I'll make gcc-4.5 the default for (at least some) architectures within the next
two weeks before more transitions start. GCC-4.5 is already used as the default
compiler for almost any other distribution, so there shouldn't be many surprises
on at least the common architectures. About 50% of the build failures exposed
by GCC-4.5 are fixed [1]. I didn't see issues on amd64 and i386, armel
(although optimized for a different processor) and powerpc (some object files
linked into shared libs had to be built as pic).

It looks like kfreebsd-* also made the switch and there's been a request
to switch for mips and mipsel.
Looking through the bug list for src:gcc-4.5, none of the open issues
seem to be specific to the remaining release architectures which haven't
switched yet - i.e. ia64, s390 and sparc. Are you aware of any issues
which would preclude switching the default on those architectures? Has
there been any discussion with the port maintainers regarding switching?

At this point, pretty well after the GCC 4.6.0 release, I would like to avoid
switching more architectures to 4.5, but rather get rid of GCC 4.5 to reduce
maintenance efforts on the debian-gcc side, even before the multiarch changes go
into unstable. I'll make GCC 4.6 the default after the release of GCC 4.5.3,
expected later this week, at least on amd64, armel, i386 and powerpc. GCC 4.6
apparently will be used for the next Fedora and OpenSuse releases, and a test
rebuild of Ubuntu natty doesn't look too bad (mostly adding new easily fixable
C++ build failures). A test rebuild of the unstable archive is still
outstanding, but these build failures will have to be fixed anyway. From my
point of view it's important to expose GCC 4.6 early in the release cycle to fix
issues like #617628 (which are issues in the packages itself) now.

With GCC 4.6 comes one soname change, bumping the libobjc version from 2 to 3,
which is not easily detachable from the GCC version change. However this change
only affects GNUstep, which can be dealt with NMU's, or migration to a new
GNUstep version.

It's unlikely that GCC 4.5 will be released with wheezy, as the Debian Ada and D
maintainers are already working on GCC 4.6 support.